In agitator mills the product generally is introduced into the milling container in the form of a suspension, flows then through the milling room during the milling operation and passes at the other end of the milling room via an outlet separator to an outlet chamber. This fluid flow also acts, of course, upon the attritive elements provided in the milling container. Even if the agitator mill is vertically arranged, the product thus flowing from below to the top through the milling container, and hence opposite to the force of gravity acting on the attritive elements, the gravity force is not sufficient to keep the attritive elements away from the separator opening. On the contrary, the elements are entrained and exert an undesirable pressure in the region of the outlet separator device. In order to obviate the unequal distribution of pressure connected therewith, it has been proposed to shorten the overall height of the milling container so that the difference of height between the inlet and outlet of the milling container is reduced. In this way the milling capacity of the mill has also been reduced and this proposal has not proved a great success. Furthermore, it has been proposed in the French Specification No. 2 015 544 to use a milling container of frustoconical shape with the inlet at the smallest end of the cone. This construction leads on the one hand to a certain improvement, on the other hand in the proposed form the particulate product is unequally treated as a consequence of varying residence or dwell times.
Agitator mills have also been proposed in German Specification Nos. 1 507 653 and 2 026 733 in which a kind of feeding screw generates a counter-pressure which provokes a downwardly directed conveying flow in the center of the milling container. As a result of this flow, the attritive elements are entrained downwardly near the screw, but outside the screw the suspended product flows upwardly so that an extremely unequal spectrum of the residence or dwell times will result.
A further problem is that the customary separating devices are unsatisfactory in some respects. By using sieve-screens there is the drawback that the openings of such screens will be clogged after a short time of operation. Therefore, slot-like separator openings have been proposed between fixed and moving surfaces or between two moving surfaces. Such areas, however, are mostly subjected to considerable wear and are thus destroyed in a short time.
East German Patent Specification No. 140 656 proposes effecting separation by arranging a rotary body opposite to a fixed wall defining the outlet opening. This rotary body causes a force which acts on the attritive elements in a radially outward direction. However, attritive elements moving along the wall of the milling container are free from the action of this centrifugal force and are, on the contrary, subjected to the pressure from below which increases towards the outlet opening and moves the elements along the fixed wall towards the outlet opening. Thus, it has not been proved possible to achieve unobjectionable separation with such an arrangement, so this proposal found no acceptance in practice.
A further problem resides in the accommodation of a separating device in agitator mills of the type which are provided with means for varying the volume of the milling room. In a known agitator mill (German Specification No. 22 40 751) the inlet for the product is formed by a central tube extending from the bottom until near the agitator where it is held and connected to the milling container (stator) by radial spokes. The piston of the varying means has a central bore with which it slides along the tube. The disadvantage is that above the orifice of the tube the spokes and the tube itself provide a quiet or calm region where product to be ground and grinding elements can accumulate without taking part in the milling operation. Moreover, when the piston moves to its highest position, it is readily possible for grinding elements to jam between the piston and the spokes whereby damage may occur. Moreover, the removal of lumpy bulks after a period of inoperation is difficult.
Furthermore, an agitator mill is known from German Specification No. 2 051 003 in which a cylinder of restricted diameter is connected to the bottom of the stator, and a vertically movable piston is arranged within said cylinder. The inlet for the product is above the orifice of the cylinder so that in this construction there is also a calm or quiet region above the piston in which the grinding elements are substantially stationary. The geometrical arrangement is such that in operation the mixture of fluid, suspended product and attritive elements is always flowable, however an accumulation of dried product and grinding elements can only be broken up with difficulty. In this case also the supply of new fluid suspended product is of no assistance in breaking up such an accumulation, because of the arrangement of the inlet above the cylinder which results in the formation of a calm or quiet region, as mentioned above.
An agitator mill is also known from U.S. Pat. No. 4,206,879 in which the displacing piston is combined with an inlet separator by providing either a peripheral slot between the piston and the wall of the container or by providing sieve openings in the piston itself. As mentioned above, such small openings tend to clogging and may become blocked.
German Specification No. 2 360 920 describes the separation of product and attritive elements at the inlet side by a swan-neck tube branching from the bottom opening of the milling container. However, it does not suggest any way of combining the swan-neck tube with the displacing piston.